I work for the NHS, currently on mat leave due to return at the beginning of December. On my maternity contract it asked how many weeks annual leave I'd like to take on my return and I put 4. This was sent to the maternity lead, signed and then in writing it says my leave was agreed by line manager. However, I have now been put down for shifts. When querying it, I was told I can't have it off due to staffing and no one is having annual leave over Christmas. Is this allowed?